The present invention relates to a color cathode ray tube having a three beam in-line type electron gun.
In general, a color cathode ray tube employs an electron gun configured to emit three in-line electron beams in a horizontal plane parallel to the major axis of a phosphor screen, and accommodated in a neck portion.
FIG. 7 is a schematic sectional view illustrating a configuration example of a prior art color cathode ray tube. In FIG. 7, reference numeral 30 indicates a panel portion; 31 is a neck portion; 32 is a funnel portion; 33 is a phosphor film; 34 is a shadow mask; 35 is a mask frame; 36 is a magnetic shield; 37 is a mask suspension mechanism; 38 is an electron gun; 39 is a deflection yoke; and 40 is a magnetic correction device. Also, reference character Bc indicates a center beam, and Bs is a side beam.
In the color cathode ray tube of this type, the panel portion 30 carrying a screen is connected to the neck portion 31 by means of the funnel portion 32, to form an evacuated envelope. The inner surface of the panel portion 30 is coated with the phosphor film 33, and the shadow mask 34 is suspended closely spaced from the phosphor film 33. The neck portion 31 accommodates the electron gun 38 for emitting three electron beams in a horizontal plane. The electron beams thus emitted from the electron gun 38 are deflected in the horizontal and vertical directions through deflection magnetic fields produced by the deflection yoke 39 disposed around the funnel portion 32, to scan the phosphor film 33, thus forming a desired image.
FIGS. 8A and 8B are views illustrating a configuration example of the electron gun accommodated in the neck portion of the color cathode ray tube shown in FIG. 7, wherein FIG. 8A is a horizontal sectional view of the electron gun, and FIG. 8B is a cross-sectional view taken along line VIIIB--VIIIB of FIG. 8A.
In FIG. 8A, reference numeral 11 indicates a heater; 12 is a cathode; 13 is a control electrode; 14 is an accelerating electrode; 15 is a focus electrode; 16 is an anode; 17 is a shield cup; 18 is an inner conductive coating coated on the inner wall of the neck portion 31; and 19 is a contact spring with one end thereof fixed on the shield cup 17 and the other end thereof pressed on the inner conductive film 18.
The operation of the color cathode ray tube accommodating the electron gun, shown in FIG. 7, will be described below.
Thermoelectrons emitted from the cathodes 12 heated by the heaters 11 are accelerated toward the control electrode 13 by the accelerating electrode 14, to form three electron beams Bc, Bs, and Bs.
These three electron beams each pass through apertures (beam-passing apertures) in the control electrode 13 and through beam-passing apertures in the accelerating electrode 14. The three electron beams are also subjected to a slight focusing action by a pre-focus lens formed between the accelerating electrode 14 and the focus electrode 15, and are accelerated by a voltage applied to the focus electrode 15 to enter a main lens formed between the focus electrode 15 and the anode 16.
The three electron beams are focused on the phosphor film 33 by the main lens, to form beam spots.
The main lens through which the side beams Bs pass is non-axially-symmetric, and it deflects the side beams Bs toward the tube axis such that the side beams Bs and the center beam Bc converge on the phosphor film 33. However, three electron beams do not converge at the center of the phosphor screen only by the structure of the electron gun because of tolerances in the manufacture of components, and accordingly they need to be converged at the center of the phosphor film 33 by adjustment of static convergence adjustment magnets for side beams and static convergence magnets for a center beam, which constitute the magnetic correction device 40.
A color image can be displayed by correctly superposing three color images of red (R), green (G) and blue (B) formed by means of three electron beams. Three electron beams are scanned over the phosphor screen through magnetic fields generated by the deflection yoke 39, to form an image.
In general, a self-converging deflection yoke is used as the deflection yoke 39.
In the case where the magnetic field of the deflection yoke is homogeneous, since the shape of the panel portion 30 carrying the screen of the cathode ray tube is not spherical with respect to the deflection center, three electron beams converged at the screen center do not stay converged when deflected.
To cope with such an inconvenience, the self-converging deflection yoke is so configured as to produce a magnetic field having an inhomogeneous distribution composed of a pin cushion-like horizontal magnetic deflection field distribution and a barrel-like vertical magnetic deflection field distribution, to obtain a self-convergence effect, thereby causing three electron beams to converge over the entire screen area.
The above-described color cathode ray tube has a disadvantage that since the magnetic field distribution of the self-convergent deflection yoke is inhomogeneous, focus characteristics deteriorate with an increasing deflection angle of electron beams and thereby resolution at the periphery of a screen is degraded as compared with that at the center of the screen.
To solve the above disadvantage, there have been known a method of applying a dynamic voltage varying with an increasing deflection angle of electron beams to a focus electrode, and a method as disclosed in Japanese Patent Laid-open No. Sho 61-250933, in which a focus electrode is composed of at least a first focus sub-electrode and a second focus sub-electrode, an electrostatic quardrupole lens is formed between facing ends of both the two focus sub-electrodes, and a dynamic voltage varying with an increasing deflection angle of electron beams is applied to the second focus sub-electrode.
The above-described methods are effective for eliminating an increase in beam spot diameter, but they distort beam spot shapes. One of causes of distortion of the beam spot shape is the shape of a panel portion being not spherical with respect to the deflection center and making the faceplate of the panel portion not perpendicular to the travelling direction of deflected electron beams.
FIGS. 9A and 9B are views illustrating the distortion of a beam spot shape associated with the shape of a panel portion of a cathode ray tube.
As shown in FIG. 9A, a spot of the electron beam on a phosphor film (or screen) forms a round shape when the electron beam is undeflected, but it forms an oval shape having the major axis along the deflection direction when the electron beam is deflected.
Accordingly, as shown in FIG. 9B, the beam spot shape on the screen is vertically elongated for vertically deflected beams, and is horizontally elongated for horizontally deflected beams.
Another cause of distortion of the beam spot shape is the inhomogeneous magnetic field distribution of the deflection yoke. The self-converging deflection yoke provides a pin cushion-like horizontal deflection magnetic field distribution shown in FIG. 10A and a barrel-like vertical deflection magnetic field distribution shown in FIG. 10B. As shown in FIGS. 10A and 10B both the deflection magnetic field distributions exert a horizontally elongating force on the deflected electron beams and thereby the beam spot shape on the screen is horizontally elongated as shown in FIG. 10C.
FIGS. 10A and 10B are views illustrating the distortion of a beam spot shape caused by a deflection yoke. FIG. 10A shows the influence of a horizontal deflection magnetic field, and FIG. 10B shows the influence of a vertical deflection magnetic field. Character X indicates the horizontal direction; Y is the vertical direction; B (vector) is a horizontal or vertical deflection magnetic field; I (vector) is the travelling direction of electron beams; and F (vector) is a force exerted on electron beams.
The actual beam spot shape on the screen is formed by a combination of the effects shown in FIGS. 9B and 10C, so that in the case of the vertically deflected beams, these effects cancel out each other to form a relatively round beam spot shape; while in the case of the horizontally deflected beams, these effects reinforce each other to form an extremely horizontally elongated spot shape.
As a result, there arises a problem that the vertical diameter of each of the beam spots at the right and left edges on the screen (phosphor film) becomes very small, thereby causing raster moire.
Moire is a phenomenon that a stripe pattern occurs on the screen due to interference between horizontal scanning lines and a periodic structure of three color phosphor dots forming the screen, to thereby degrade resolution. When the beam spot diameter becomes smaller than a value determined by the periodic structure of the phosphor dots, moire is caused.